In the drilling process in deep wells and ultra deep wells, clay and drill cuttings in the drilling fluid are dispersed at high temperature, since the drilling fluid is under high temperature condition for a long time; consequently, the drilling fluid is thickened at high temperature, resulting in increased viscosity and degraded rheological property of the drilling fluid. As oil and gas exploration evolves to deeper formations, the encountered formations are more and more complex, and the requirements for drilling fluid become higher and higher. In view of this problem, a series of research work has been done on high temperature viscosity reducers for drilling fluid in the countries around the world, centering on the requirements for well drilling in ultra deep wells. Those researches mainly focused on synthetic polymers (e.g., acrylic acid and vinylsulfonic acid, etc.) and organic phosphates, and these viscosity reducers have outstanding temperature tolerance performance. CN101928366A discloses a method for preparation of viscosity reducer for drilling fluid, comprising the following steps: dissolving a mixture of reactant monomers acrylic acid (AA) and 2-acrylamide-2-methyl propane sulfonic acid (AMPS) mixed at 0.5-10:1 mix ratio (mass ratio) in water, dissolving ammonium persulfate in amount equal to 0.5-5% total mass of the monomers in water, dissolving two molecular weight regulators in amount equal to 0.3-5% total mass of the monomers respectively in water and then loading into a reactor and heating up to 55-65° C., adding the solution of the reactant monomers and the solution of ammonium persulfate into the reactor, and then heating up to 85-95° C. and maintaining the reaction for 1 h, to obtain a viscosity reducer with 10-50% content of the monomers; neutralizing the system with sodium hydroxide to pH 7 after the reaction, and drying and crushing the product, to obtain a viscosity reducer for drilling fluid that can withstand 220° C. or higher temperature. However, the viscosity reducer for drilling fluid can't meet the requirements for actual application, since the viscosity reducing effect of polymers that contain carboxyl and sulfonic acid will be degraded in salt water drilling fluids and high-density drilling fluids, and organic phosphates have drawbacks such as short maintenance interval and narrow range of application, etc. Therefore, it is urgent task to develop a viscosity reducer for drilling fluid, which has good viscosity reducing effect after high temperature aging in salt water drilling fluids and high-density drilling fluids.